Entropy Enhanced Perovskite Oxide Ceramic for Efficient Electrochemical Reduction of Oxygen to Hydrogen Peroxide

Abstract The electrochemical oxygen reduction reaction (ORR) offers a most promising and efficient route to produce hydrogen peroxide (H2O2), yet the lack of cost‐effective and high‐performance electrocatalysts have restricted its practical application. Herein, an entropy‐enhancement strategy has been employed to enable the low‐cost perovskite oxide to effectively catalyze the electrosynthesis of H2O2. The optimized Pb(NiWMnNbZrTi)1/6O3 ceramic is available on a kilogram‐scale and displays commendable ORR activity in alkaline media with high selectivity over 91 % across the wide potential range for H2O2 including an outstanding degradation property for organic dyes through the Fenton process. The exceptional performance of this perovskite oxide is attributed to the entropy stabilization‐induced polymorphic transformation assuring the robust structural stability, decreased charge mobility as well as synergistic catalytic effects which we confirm using advanced in situ Raman, transient photovoltage, Rietveld refinement as well as finite elemental analysis.

3 linear confinement for the occupation of the multiple-metal atom was carried out on the basis of compositional analysis result. The element chemical states of samples were characterized by X-ray photoelectron spectrometry. The XPS measurements were carried out on an ESCALAB 250Xi spectrometer (Thermo Scientific, USA) equipped with a pass energy of 30 eV with a power of 100 W (10 kV and 10 mA) and a mono-chromatized AlKα X-ray (hν=1486.65 eV) source. All samples were analyzed under a pressure of less than 1.0×10 -9 Pa. Spectra were acquired through the avantage software (Version 5.979) with a step of 0.05 еV. The metal contents in the samples were determined by using an inductively coupled plasma optical emission spectrometer (ICP-OES), (Varian 720-ES). The morphology and microstructure of samples were investigated by Field-emission scanning electron microscope (FESEM, Zeiss G500) and transmission electron microscope (TEM, FEI Talos F200X). In-situ Raman spectra during the ORR process were recorded by a Raman spectrometer with an excitation wavelength of 532 nm (Horiba LabRAM HR Evolution).
Electrochemical test: First, 3.6 mg of as-prepared electrocatalyst and 0.4 mg of Ketjen Black are mixed and dispersed in a mixture solution containing 600 μL H2O, 300 μL ethanol, and 100 μL 0.5 wt.% Nafion solution. In order to obtain a uniform ink, the mixed solution was sonicated in an ice water bath for 30 min. Then 6.28 μL of the ultrasonic solution was dripped on the glassy carbon of the rotating ring disk electrode (RRDE) with an area of 0.1256 cm 2 . An electrochemical workstation (760E, CHI) was used to evaluate the electrochemical performance of the catalysts. A standard three-electrode system was used to evaluate the performance of the catalyst, where the RRDE loaded with catalysts, saturated calomel electrode, and graphite rod are served as the working electrode, reference electrode, and counter electrode, respectively.
The electron transfer number (n) and H2O2 selectivity (H2O2%) are calculated by the followings, where IR represents the ring current, ID represents the absolute value of disk current and the collection efficiency (N) of the RRDE is equal to 0.37.
H-cell electrolyzer and organic dyes degradation: The H2O2 yield of the catalyst in 0.1 M KOH was determined using an H-cell electrolyzer, in which the catalyst-loaded carbon paper was used as the working electrode, and the commercial IrO2 loaded carbon paper was used as the counter electrode, and the mass loading of the catalysts both are 0.5 mg cm -2 . The catholyte containing HO2is taken out and acidified by adding sulfuric acid. The H2O2 yield determination was quantified through the reaction between cerium sulfate (CeSO4) and H2O2: 2Ce 4+ + H2O2 → 2Ce 3+ + 2H + +O2. The relationship between the concentration of Ce 4+ and absorbance was calibrated by ultraviolet-visible spectrum (UV-vis). Three organic dyes with a concentration of 30 ppm were prepared, namely rhodamine B, methyl orange, and methylene blue. Firstly, 5 mL of catholyte is collected from the cathode area after discharging the electrolytic cell at a current density of 35 mA cm -2 for 30 min. Secondly, 1 mL 1 M H2SO4 and 1 × 10 -3 Fe 2+ were added into the obtained 5 mL catholyte to acidizing the solution. Finally, the above-acidified solution containing Fe 2+ was mixed with 10 mL of dyes solution, shaking continuously to ensure a sufficient reaction (Fenton reaction). Dyes are degraded by hydroxyl radicals (OH⸱) generated by Fenton reaction: 2Fe 2+ + H2O2 → 2Fe 3+ + OH -+ OH⸱. The concentration of the dyes at different moments is determined using a UV-Vis, and the absorption peaks of the three dyes are located at 664 nm (methylene blue), 463 nm (methyl orange), and 553 nm (rhodamine b), respectively.
TPV principle: A stimulus response method, called transient phototelepressure (TPV) measurement, was performed on a homemade measurement system with a platinum net covering a power sample (1 cm * 1 cm) as the working electrode and a Pt line as the counter electrode sample at room temperature from a third harmonic Nd: YAG laser (Polaris II, New) Wave Research, Inc.) laser radiation pulse (λ = 355 nm, pulse width 5 ns) excited TPV signal is first amplified, and then the oscilloscope records the photocurrent is the ratio of the photovoltage to the internal resistance of the test system.

Finite Element Analysis:
The finite element analysis (FEA) can simulate various physical and chemical properties/reaction processes at mesoscopic scale, thus being widely employed in different research fields, including mechanics, fluids, electromagnetics, optics, and electrochemistry [S2]. More intriguingly, the FEA simulation can simplify the complexity of the reaction models to reflect the general trend, especially at the dynamic proceeding where vague and changeable intermediates are involved. Consequently, during the electrochemistry, the application of FEA can shed more lights on the reactions at the interface between the electrode and electrolyte, which are typically neglected before. This is because the dominant theoretical calculation in this area is based on the first-principle method (e.g., density functional theory, DFT), which is confined at modeling atomic configurations (revolving around the specific active atomic sites) where the energetics of elementary reaction processes are calculated. Furthermore, the influences resulting from the externally applied electric field and structural evolution of electrodes at longer timescale which are beyond the DFT calculation regime can also be captured by FEA [S3]. In the specific area of electrocatalysis, the FEA was more extensively utilized to simulate the properties, such as electric field distribution, direction, and intensity, of the surface of the electrode, which then can estimate the surrounding concentration of the reactants or intermediate ions, thereby reflecting the catalysis kinetics [S2f, 4]. Similarly, in the conventional electrochemical cell, the pH value and potential in the immediate vicinity of the electrode can also be assessed [S5]. Based on these, now it can be observed that FEA methods emerged in different electrolysis, including hydrogen evolution reaction [S6], oxygen evolution reaction [S4], carbon dioxide reaction [S2f], providing deeper insights into the catalysis mechanisms. The stress, electrical field, and charge distribution for 6 the Pb(ZrTi)1/2O3 and Pb(NiWMnNbZrTi)1/6O3 particle during ORR were modeled by the COMSOL Multiphysics® 5.5 software which was based on the Finite Element Analysis. [S7] Due to the high calculation and complexity of the actual three-dimensional model, the actual three-dimensional model is reasonably simplified to a two-dimensional model under the premise of fully ensuring that it is consistent with the actual physical process. [S8] The particle size was set as 2 µm, and the length and width of the electrode substrate was set as 50 and 5 µm, respectively ( Figure S29). After the geometric model was successfully created, the material property was assigned according to the actual experimental settings. Specifically, the piezoelectricity constant and relative permittivity for Pb(NiWMnNbZrTi)1/6O3 were set as the 0 pC N -1 and 1300, respectively, while those for Pb(ZrTi)1/2O3 were set as the 520 pC N -1 (isotropy) and 2400, respectively. The actual physical process involved the three-field coupling of fluid field, electrostatic field and solid mechanic field. In the experiment, the material was in a high-speed flowing fluid, so a fluid field was applied to simulate the actual experimental process. The specific settings of the fluid field were shown in Table S8. The electrostatic field was a description of the electrostatic response of the model. Because the actual electrochemical reaction process was quite complicated, a change in the surface potential of 0.7 V vs. RHE was applied to the powder to simplify the expression. The reason why the potential was selected is due to the thermodynamically theoretical voltage of producing peroxide hydrogen. The mechanical response of structural materials was simulated by the solid mechanic field.
The speed f of the sample and electrode in the electrolyte solution was set to 1600 r min -1 , the radius of rotation r is set to 5 cm, and the steady-state average flow velocity U (8.38 m s -1 ) at the inlet could be solved according to the following formula: The mesh division was automatically performed through the software, and the division result is shown in Figure S30.

Description of XPS spectra of Pb(ZrTi)1/2O3
The high-resolution XPS spectra of Pb(ZrTi)1/2O3 were shown in Figure S2.  Figure   S2b). In contrast, the Ti-O bonds showed in Figure S2c shifted towards higher binding energy when entropy was increased. Moreover, the high-resolution O 1s XPS spectra in Pb(ZrTi)1/2O3 also showed a negative shift than that of Pb(NiWMnNbZrTi)1/6O3 ( Figure S2d).